Two parallel packs made from different C rate cells.

Buk___

10 kW
Joined
Jul 28, 2017
Messages
750
If I parallel two 10s4p packs made from different cells, samsung 26H & 29F, which are max rated as 5.2Ah (x4=20.8 ) and 8.25Ah (x4=33) respectively, and I occasionally draw 25A for short periods; what happens?

Update: With more reading and thought I now think that the instantaneous demand will draw equally from both packs; but the lesser pack will go to a lower voltage, at which point there will be a net flow from the larger to the smaller until their voltages equalise.

Does that make sense? Can I treat the combined pack as a single 22Ah and ignore the differences?


I see several possibilities and I have no idea which is correct:

1) Most of the draw will come from the higher C-rate pack and the lesser one will make up the shortfall.

2) The lower C-rate pack will supply it's max, and the rest will be drawn from the higher rate pack.

3) 25A will be drawn from both packs, quickly wrecking the lower rate pack.

4) 12.5A will come from each pack until the power rate pack runs out of juice.
 
#2 is closest to what happens, but it will only max out if the amps you are pulling from both is very high, beyond what spec says would be the max for both. Your combined packs should tolerate 25 amps ok. In most cases, you start to add up parallel batteries, and both will be much lower than their max spec. That's the big benefit of paralleling, when the ah gets so big your discharge is more like .5c. Then you get very low sag under load, and the battery stays very happy.

What will happen is more amps will be drawn from the pack that sags the least under load. Its internal resistance under load is less, and more will flow from that pack because its having less resistance to doing that.

Which pack carries the most load will depend on both c rate and size of each pack. If they are close to the same size, and your amps drawn is not that high, then it may be that neither pack sags much under load, and both get the same load.

If the load is off long enough, the now higher voltage, more full high resistance battery will start to charge the other one back up. Then when you whack the throttle, the sags less battery will again take up the load more.

Chances are, your batteries are still similar enough, even if pretty different in Ah, that in practice you will not have anything extreme going on. Any time you stop, unplug, and check, you won't find much difference in voltage. The bigger, or better cell pack will not ever really be discharging deeper than the other one. The benefit of sharing the load will still help the weak pack stand the discharge, even though its discharge is kind of constant. It will discharge when under load, and then still push some current into the stronger battery when the load is off. Both still lower than if it ran by itself, so its all good.

It will though have an extreme charge into the lower voltage battery, if you aren't close enough to the same voltage when you plug in, like within one volt. So charge them up before the parallel connection is made.

What can cause a problem is if the chemistry is completely different, and the voltage of the packs at empty is very different. Like limn and lipo for example. One is empty at 3.5v and the other keeps going lower. Your case is not like that, I just mention it because I have done that sort of thing. I had to be careful to unplug the lipo before it ran down too much. ( no bms on the lipo)
 
dogman dan said:
Chances are, your batteries are still similar enough, even if pretty different in Ah, that in practice you will not have anything extreme going on. Any time you stop, unplug, and check, you won't find much difference in voltage. The bigger, or better cell pack will not ever really be discharging deeper than the other one. The benefit of sharing the load will still help the weak pack stand the discharge, even though its discharge is kind of constant. It will discharge when under load, and then still push some current into the stronger battery when the load is off. Both still lower than if it ran by itself, so its all good.

It will though have an extreme charge into the lower voltage battery, if you aren't close enough to the same voltage when you plug in, like within one volt. So charge them up before the parallel connection is made.

Great. That's what I needed to hear. Thanks for the feedback Dan.
 
#1 and #2 is ambiguous language, but Dan is right. The lower resistance pack (higher C rate) will provide most power, with the lower C rate sagging and tipping in what it can. When the load is off, the lower C rate pack tops the higher one back up. In extreme cases, becauss the battery itself is providing the only resistance/load regulation, this can be a very high current.

I paralleled a very large 300mOhm old battery with a much smaller but newer 30mOhm battery. During flat riding (Drawing up to 40a) the pack remains completely balanced. When hill climbing, I can sustain over 100a for 2-3 minutes. when the bike finally cones to a rest, I can see the pack voltages slowly rise for quite some time, (up to 30s?)as the bigger older LiFePo4s top up the drained LiPos.
 
Its amazing how well this works, even with wildly different capacities, or c rates. But to work great, they need to be the same chemisty.

Mostly I've done this with very old very tired, barely functioning lipo. Used alone, 50% capacity at best. But parallel with a strong pack, and you can get 70% again. Keeping that c rate on the beat up puffed up pack to .25c is the key. Amazing what is still in there, if you trickle it out slow enough.

lifepo4 with lipo, or lipo with limn, then you get into very different voltage at 90% discharged, and have to be really careful if packs do not have a bms to shut them off when empty.
 
Back
Top